1. Field of the Invention
The present invention relates to an electromagnetic wave absorber which can be used in various fields, e.g., the electronic/electrical field of various kinds of electronic apparatuses such as a computer and a television set, communication apparatuses, etc., the constructional material field of shielded buildings, shielded facilities, etc., and the field of human body protecting goods.
2. Description of the Related Art
It is an urgent necessity to take measures against electromagnetic interference with the recent remarkable advance of performance of various kinds of electronic apparatuses, communication apparatuses, etc. Specifically, examples of electromagnetic interference include a ghosting phenomenon of a television set caused by electric wave reflected by a high-rise building, a mirror image phenomenon of a radar disturbing the operation of an automobile, a train, etc., and a wrong operation of medical equipment caused by electric wave issued from a mobile communication apparatus such as a portable telephone set.
Further, these apparatuses are defenseless against TEMPEST (information technique by which electromagnetic wave leaked from a personal computer or the like can be monitored at a place far by 100 m or more).
A substrate such as a foamed urethane resin impregnated or coated with a carbon material is used popularly as a conventional electromagnetic wave absorber. Absorption of electromagnetic wave is performed in such a manner that the electromagnetic wave absorbing material such as a carbon material absorbs electromagnetic wave and converts electromagnetic energy of the electromagnetic wave into heat energy. On this occasion, the electromagnetic wave absorbing material generates heat. For this reason, when strong electromagnetic wave is received, the quantity of heat generated in the electromagnetic wave absorbing material becomes large. Hence, there is a problem that the foamed urethane resin as the substrate may be deformed thermally, melted or ignited (explosively burned at 600xc2x0 C. or higher) or may produce a poisonous gas.
For example, as shown in FIG. 40, an electromagnetic wave absorber 100 used in an electromagnetic anechoic chamber has a base 101, and a large number of pyramidal portions 102 provided on the base 101. The height H of the electromagnetic wave absorber 100 is from the order of tens of cm to the order of m. For this reason, the space occupied by the electromagnetic wave absorber 100 in the anechoic chamber is so large that the space utilization factor in the chamber is worsened. Moreover, the electromagnetic wave absorber 100 is thick and heavy. In addition, there is a disadvantage that the electromagnetic wave absorber 100 is combustible as described above.
Moreover, the technical field requiring such an electromagnetic wave absorber has extended over a very broad range in recent years. The electromagnetic wave absorber is applied to a range of from the frequency band of KHz to the frequency band of GHz of millimeter wave.
In order to eliminate the disadvantages in the related art, an object of the invention is to provide an electromagnetic wave absorber which is high in electromagnetic wave absorbing performance, which can be made thin and lightweight and which is so excellent in reliability that the electromagnetic wave absorbing performance can be offered over a very broad range.
In order to achieve the object, according to first means of the invention, there is provided an electromagnetic wave absorber including: a porous substrate having a large number of pores piercing the porous substrate; and an absorbent film formed on circumferential surfaces of the pores and constituted by a mixture of an electromagnetic wave absorbing filler and an electromagnetic wave absorbing high-molecular material, wherein the pores are not blocked with the absorbent film so that the pores are permeable to gas.
According to second means of the invention, there is provided an electromagnetic wave absorber including: a porous substrate having a large number of pores piercing the porous substrate; and incombustible or flame-retardant particles having pores into which filamentary carbon adheres, the pores of the porous substrate being filled with the incombustible or flame-retardant particles.
According to third means of the invention, there is provided an electromagnetic wave absorber including: a porous substrate having a large number of pores piercing the porous substrate; an absorbent film formed on circumferential surfaces of the pores and constituted by a mixture of an electromagnetic wave absorbing filler and an electromagnetic wave absorbing high-molecular material; and incombustible or flame-retardant particles having pores into which filamentary carbon adheres, the absorbent film-coated pores of the porous substrate being filled with the incombustible or flame-retardant particles.
According to fourth means of the invention, there is provided an electromagnetic wave absorber as defined in any one of the first through third means, wherein the porous substrate is made of either of an electromagnetic wave shielding material and an electromagnetic wave absorbing material.
According to fifth means of the invention, there is provided an electromagnetic wave absorber as defined in any one of the first through third means, wherein the absorbent film is constituted by a laminate of layers of absorbent films different in electromagnetic wave absorbing filler content.
According to sixth means of the invention, there is provided an electromagnetic wave absorber as defined in any one of the first through third means, further including a thin layer containing electromagnetic wave absorbing metal or carbon and provided so as to overlap the absorbent film.
According to seventh means of the invention, there is provided an electromagnetic wave absorber as defined in any one of the first through sixth means, wherein the porous substrate has a honeycomb structure.
According to eighth means of the invention, there is provided an electromagnetic wave absorber as defined in any one of the first through seventh means, wherein an axial direction of each of the pores in the porous substrate is inclined to a plane of the porous substrate.
According to ninth means of the invention, there is provided an electromagnetic wave absorber as defined in any one of the first through third means, wherein the electromagnetic wave absorbing high-molecular material is a modified polyester resin constituted by a copolymer of isobutyl methacrylate and butyl acrylate.
According to tenth means of the invention, there is provided an electromagnetic wave absorber as defined in any one of the first through ninth means, wherein the electromagnetic wave absorber is constituted by a laminate of sheet-like electromagnetic wave absorbers.
According to eleventh means of the invention, there is provided an electromagnetic wave absorber as defined in the tenth means, the electromagnetic wave absorbers to be laminated are integrally bonded to one another by a modified polyester resin constituted by a copolymer of isobutyl methacrylate and butyl acrylate.
According to twelfth means of the invention, there is provided an electromagnetic wave absorber as defined in any one of the first through ninth means, wherein the electromagnetic wave absorber is coated with incombustible or flame-retardant particles having pores into which filamentary carbon adheres.
According to thirteenth means of the invention, there is provided an electromagnetic wave absorber as defined in any one of the first through ninth means, wherein: the electromagnetic wave absorber is constituted by a laminate of sheet-like electromagnetic wave absorbers; and incombustible or flame-retardant particles having pores into which filamentary carbon adheres are interposed between the sheet-like electromagnetic wave absorbers.
According to fourteenth means of the invention, there is provided an electromagnetic wave absorber as defined in any one of the first through ninth means, wherein: the electromagnetic wave absorber is processed three-dimensionally; and a hollow portion of the processed electromagnetic wave absorber is filled with incombustible or flame-retardant particles having pores into which filamentary carbon adheres.
According to fifteenth means of the invention, there is provided an electromagnetic wave absorber as defined in any one of the first through fourteenth means, wherein the electromagnetic wave absorber is disposed in the periphery of a printed wiring board mounted with electronic parts or in the periphery of electronic parts.
According to sixteenth means of the invention, there is provided an electromagnetic wave absorber as defined in any one of the first through fourteenth means, wherein the electromagnetic wave absorber is used as a constructional material, for example, for constructing a side wall, a ceiling wall, a floor wall, and a partition wall.